1. Field of the Invention
The present invention relates to an apparatus for indicating temperature at several different values. In particular, a thermochromic semiconductor is utilized which changes its color or absorbance in response to temperature. This color change provides an indication of danger and the degree of danger to an individual. The United States Government has rights in this invention pursuant to Contract No. DE-AC09-89R18035 between the United States Department Of Energy and Westinghouse Savannah River Company.
2. Discussion of Background
Burn injuries to human skin and other tissues can occur at temperatures above 100 degrees Celsius (212 degrees Fahrenheit) and occasionally at temperatures as low as 60 degrees Celsius (140 degrees Fahrenheit). Many human activities involve objects or surfaces heated into this range. Range top cooking, for example, uses a hot stove eye or griddle surface. Electronic component assembly often requires a heated soldering iron or soldering gun. In some cases these surfaces are hot enough to glow, providing a built-in warning of high temperature. However, incandescence bright enough to be seen in normal room lighting appears only when the temperature is between 500 to 700 degrees Celsius. Where the heated objects or surfaces have temperatures above 60 degrees Celsius but below noticeable incandescence, there is a potential for bum injury yet no obvious visible sign warns of this danger.
A number of temperature indicating devices have been devised to warn of high temperatures. The most familiar are glass thermometers which usually contain mercury, dyed alcohol or liquid gallium alloys. These thermometers are not practical for measuring the temperature of surfaces because they are mechanically fragile. Dial thermometers, usually driven by bimetallic strips, are more rugged but still subject to damage or calibration loss because of their mechanical complexity. Thermocouples and thermistors are still more rugged, but the electronics needed to linearize and display the temperature of such a device is too costly and bulky for many applications.
Other materials which may be used to indicate temperature are those which melt or soften predictably with temperature. One example is pyrometric cones which are used chiefly in ceramics. These cones are compounds of clays and glassy materials which soften and bend over at known temperatures ranging upward from about 600 degrees Celsius. More recently, materials which melt at lower temperatures have been sold commercially by the name Stik.TM.. These Stiks.TM. look chalky up to their melting points and become glossy at higher temperatures. Up to about 150 degrees Celsius, such materials may also be used to make self-adhesive labels with dots which change color, usually from white to black, when they melt. Once a cone, Stik.TM. or label dot has melted, the indication is permanent and the temperature indicator cannot be reused.
Another group of usable temperature indicating materials are those which change their colors, rather than their shapes or mechanical properties, with temperature. These materials are called thermochromic. The most familiar thermochromic material is liquid crystals which can be formulated to respond over narrow temperature ranges from 0 to 60 degrees Celsius. Liquid crystals are synthetic derivatives of cholesterol. Liquid crystals are intrinsically colorless, but the ordering causes a response like three-dimensional diffraction gratings.
Cupromercuric iodide is another thermochromic material which undergoes a change in crystal form and hence in color at or near 67 degrees Celsius. The red form is stable below 67 degrees Celsius and the second, brown form is stable above that temperature.
Both liquid crystals and cupromercuric iodide provide reversible temperature indications within their normal temperature ranges, but are readily decomposed by temperatures very much above their operating ranges. Therefore, these indicators are unusable for providing a warning for most surfaces which may become hot enough to cause burns.
A more detailed description of various thermochromic materials is given in pending U.S. patent application Ser. No. 08/332,068, filed Nov. 1, 1994, entitled Optical Temperature Sensor Using Thermochromic Semiconductors (S-79,559) which is hereby incorporated by reference.